This invention relates to a voltage and frequency control circuit for converting an input ac voltage with inconstant frequency and amplitude into an ac voltage with constant amplitude and a constant nominal frequency which is lower than the lowest occurring frequency of the input ac voltage.
When using generators whose speed is not controlled, both the amplitude as well as the frequency of the ac voltage which is produced is not constant. In this case of a generator with a relatively low output, e.g. a generator driven by a windimill, it is neither economical nor expedient to provide control equipment for maintaining the frequency and amplitude at a constant level, as it is already known in the case of generators which are employed in large power stations. Frequently, the ac voltage produces has also a substantially higher frequency than the conventional main frequency of 50 or 60 cps. For instance, when converting the wind energy in windmills using generators suitable for this purpose, ac voltages occur which have frequencies ranging from approximately 300 to 800 cps which deviate by up to 40% from a mean frequency according to the rotational speed at the moment. The deviations of the amplitude from a mean amplitude value also are of this same magnitude.
The object of the present invention is to provide a simple voltage and frequency control circuit for converting the frequency as well as the amplitude of an inconstant ac voltage into an ac voltage with a constant predetermined frequency and amplitude.
This object is accomplished in accordance with the present invention in that the input ac voltage to be converted is supplied to a control circuit consisting of a series of switching units including an amplitude control circuit, a regulatable rectifier circuit and a filter circuit. The amplitude control circuit is designed such that it converts an ac voltage of inconstant amplitude on the input side into an ac voltage of constant amplitude on the output side. The regulatable rectifier circuit is regulated by an ac voltage source which supplies a control ac voltage with nominal frequency such that in the half-cycles in which the control ac voltage has positive polarity, half-waves of the input ac voltage are produced with a first polarity and in the half-cycles in which the control ac voltage has negative polarity, half-waves are produced with a second polarity. The filter circuit is provided such that at least approximate smoothing and sine wave shaping occurs.
In a preferred embodiment of the rectifier circuit of the present invention, the input ac voltage whose frequency exceeds the frequency of the desired output ac voltage (50 to 60 cps) is rectified such that the positive and negative half-waves of the input ac voltage have a first polarity during the positive half-waves of a control ac voltage with a frequency equal to that of the desired output ac voltage. Similarly, the positive and negative half-waves of the input voltage have a second polarity opposite to the first polarity during the negative half-waves of the control voltage. Assuming a control ac voltage of 50 cps (period of 20 msec), half-waves with positive or negative amplitude are produced by this rectification during successive intervals of 10 msec, respectively. In the amplitude control circuit unit, either the input ac voltage or the already rectified ac voltage is converted into an output ac voltage with constant amplitude. In addition, the filter circuit is provided for producing the rectified ac voltage in an approximately sinusoidal ac voltage having the desired frequency, e.g. 50 cps. It is also possible in principle to provide the amplitude control circuit directly in front of the output of the entire control circuit, i.e. to rectify the input ac voltage first, then permit it to pass through a filter and finally, stabilize the amplitude of the resultant output voltage. Since an increase in generator speed results simultaneously in an increase in frequency as well as in voltage and in the output of the ac voltage which is produced, the present invention has the advantage that the transformer employed is always dimensioned optimally, since at higher frequencies higher outputs can be transferred.
Further advantageous features are described in the claims.